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How to sharpen centre drills?

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marcusj17/03/2010 16:07:53
27 forum posts
I have Harold Hall's excellent book 'Tool and Cutter Sharpening' but I'm left with the question: how do I sharpen my hard working centre drills?
Michael Wilde24/04/2010 23:12:22
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25 forum posts
16 photos
Hi, I also have this book and I'm currently making the grinding rest described within. I'm not sure about sharpening centre drills, maybe if you had one of the common swing type drill grinding attachments that are also described in Harolds book you could rig it up to hold a centre drill? Remember the projection from the front of the attachment is specific to the diameter of drill being sharpened so this would be the pilot diameter of the centre drill. I'm pretty sure the angle of the pilot tip of a centre drill is 60 degrees aswell as the conical, centre forming section, but I could be wrong... Also, you can't shorten the pilot section too much as the length is needed as clearance for the tip of your tailstock centre once an impression has been made with the centre drill. I broke a centre drill today (whilst making the fence for the grinding rest). The pilot of the centre was well bedded in to the material so I marked out for the hole on the opposite side, centre popped, centre drilled and then very gently drilled until the drill felt as though it wasn't cutting. I placed the material in the bench vice and centre punched the bottom of this blind hole. This punched the broken section of centre drill out the other side! Beats trying to 'chisel' it out with old scribers! Sometimes trying a magnet is useful aswell if it's not stuck in too tight.
Michael Wilde24/04/2010 23:42:37
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25 forum posts
16 photos
Hi, just been googling centre drill geometry. I found this .PDF wich is quite useful. http://www.shis.com.sg/product/100.pdf with reference to my above post about the pilot angle being 60 degrees: it's not! It's 120 degrees. Should make it a bit easier to set in drill grinding attachment. If a length of square material was drilled/reamed as accurately as possible on one face to accept the centre drill body, then a grub screw fitted perpendicular to this to clamp the centre drill. It would be possible to set this up on the drill grinding attachment with the relevant projection and the pilot flutes in a vertical plane. The square material would sit nicely into the 'V' of the attachment and would allow it to be turned exactly 180 to grind the second flute to the same setting as the first. I don't know if this would work in practice, it's just a thought. There's probably many other methods but it's all I can think of right now. Let me know how it's going if you attempt this! Cheers, Mike
chris stephens25/04/2010 12:27:16
1049 forum posts
1 photos
Hi Micheal,
Centre drills tend to break their tip when you don't withdraw the drill often enough, whilst drilling. The tip is very fragile and any flute clogging is likely to snap the end off.
chriStephens
Stub Mandrel27/04/2010 21:28:39
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4318 forum posts
291 photos
1 articles
I have an ocassional problem when centre drilling from the tailstock - once in a blue moon* the drill wanders off centre and creates a circuilar groove, and forcing it snaps the drill
 
Keeping a close eye on things and forcing the drill back on centre can cure it just as it starts, if I am quick. I think the cause is an over-extended, unlocked tailstock.
 
Neil
 
*my cussing turns the air blue too...
Michael Wilde27/04/2010 22:56:59
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25 forum posts
16 photos
I used to have the same problem when centre drilling/drilling. I thought it might have been the tailstock out of align but when checked was not so. I gripped a rod in the drill chuck, gripped the other end in the 4 jaw chuck, put a clock on the morse taper of the drill chuck and rotated the lathe spindle by hand. An error of about 3 to 4 thou (0.075mm to 0.1mm) was found. The drill chuck and morse taper was a cheap set-up, I ordered a new chuck and MT and the problem was solved!
KWIL28/04/2010 08:31:50
3681 forum posts
70 photos
I usually find that by the time the tip needs a regrind the main cone angle also needs attention, so its just a spare bit of HSS now and all I need is a new one. Life is much too short to regrind centre drills.
Ian S C28/04/2010 12:12:20
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7468 forum posts
230 photos
I just sharpen my worn out/broken center drills as stub drills, countersink bits, or use them as tool steel for boring bars. Ian S C

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